The present invention relates to tires suitable for heavy loads and to the use of a rubber composition in treads for such tires, where such composition leads to a delay in the appearance of irregular wear on the treads during travel. The present invention is applicable to tires for heavy loads, such as on motor vehicles, heavy vehicles, construction vehicles and aircraft.
The wear resistance of a tire that bears heavy loads is evaluated by measuring the weight loss of this tire relative to a control tire and following travel over the same distance, which is sufficient to involve a significant weight loss. Independent of the wear resistance, “moment of appearance of the irregular wear on a tread” is conventionally understood to mean the moment at which the gradient of the characteristic “loss of weight of rubber as a function of traveling distance” increases substantially. This gradient is sometimes referred to as “weight loss rate”. This sudden change in gradient coincides with the formation of specific facies of irregular wear on the radially outer face of the tread.
The facies of irregular wear may be characterized, on one hand, by a very high weight loss rate at the location of the edges of the central ribs which are located close to the median circumferential plane of the tread and, on the other hand, by a weight loss rate at the location of the ribs of the “shoulder” zone of the tread which is much higher than that relating to the central ribs.
Various methods have been attempted to delay the appearance of irregular wear in a tire tread bearing heavy loads. A first method involved acting on the forms of the tread patterns, as described, for example, in patent specifications JP-A-99/105 513, JP-A-97/175 120 or EP-A-705 721. A second method involved producing a tread in two parts, as described in U.S. Pat. No. 4,360,049.
As provided in U.S. Pat. No. 4,360,049, a radially outer part (i.e. one intended to be in contact with the ground) is formed from a rubber composition, the elastomeric matrix of which comprises primarily a copolymer of styrene and butadiene, with carbon black as reinforcing filler. This radially outer part provides the corresponding tread with irregular wear which is satisfactorily delayed. Moreover, the tread is satisfactorily resistant to attack due to travel. However, the rubber composition used in this outer part has high hysteresis losses.
This is the reason that a radially inner part for the tread is provided, which is in contact with the outer protective crown ply. This radially inner part is formed from a composition having relatively low hysteresis losses, the effect of which is to limit the internal heating of the tread during travel and, consequently, limit the rolling resistance of the tire.
A third method provides a tread characterized by the specific formulation of the rubber composition which constitutes the tread.
First, rubber compositions for use in treads comprised an elastomeric matrix based on natural rubber, which has lower hysteresis, with carbon black as reinforcing filler. However, such compositions provide relatively reduced adhesion to treads comprising such compositions.
A further approach was to replace the aforementioned compositions with compositions comprising an elastomeric matrix based on a copolymer of a conjugated diene monomer and a vinyl-aromatic monomer, such as a copolymer of styrene and butadiene, reinforced with carbon black. Such compositions impart to the corresponding treads satisfactory wear resistance, delay in irregular wear, and good resistance to attack due to travel. However, these compositions have very high hysteresis losses.
Improvements included adding to the copolymer an active function at the chain end for coupling to the carbon black. The compositions thus obtained had hysteresis losses which are substantially equivalent to those of natural rubber. However, such compositions impart to the tread insufficient resistance to attack. Moreover, they do not make it possible to delay the irregular wear or to provide satisfactory wear resistance in the treads.
In order to overcome all the aforementioned disadvantages, in particular the hysteresis losses, the wear resistance and the irregular wear, attempts have been made recently to replace the above traditional tread compositions with those that use silica as the primary reinforcing filler instead of carbon black.
The reinforcement of rubber compositions comprising natural rubber or a copolymer of a conjugated diene and a vinyl-aromatic compound prepared in solution, with silica as the major reinforcing filler provides hysteresis losses which are relatively low for the corresponding compositions. However, such compositions do not make it possible to satisfactorily delay the irregular wear for treads formed from these compositions. Moreover, the wear resistance is unsatisfactory.
It should be noted that the use of a silica reinforcing filler results in compositions comprising a synthetic polymer, which are difficult to process. Furthermore, the cost of preparing these compositions is relatively high, because of the requirement for a silica/elastomer bonding agent.
The use of a carbon black/silica blend as reinforcing filler provides the same aforementioned disadvantages, in relation to filler based on carbon black or silica in the relative mass fractions which are used.
The inventor has unexpectedly discovered an improved cross-linkable rubber composition that can be used to form the entire tread of a tire used for bearing heavy loads.